Re: Is PAM II working optimally?

‎2012-08-2106:42 AM

Hi Casey,

Here's some info from memory...

Firstly, the cache hit rate shown in sysstat output isn't showing the same thing as the flexscale stats. Ignore the sysstat one for now...

With a low hit rate you should look into what type of data you are missing, and what you are caching. If you use stats show ext_cache_obj this will give you more info. Better still run it over a few minutes using stats start -I cache ext_cache_obj ...wait for a minute or two during a busy period... stats stop -I cache.

This will give an output like this (notes added for some counters to check):

StatisticsID: cache

ext_cache_obj:ec0:type:IOMEM-FLASH

ext_cache_obj:ec0:blocks:67108864

ext_cache_obj:ec0:size:256 <------- size of cache in GB

ext_cache_obj:ec0:usage:92% <------- How full the cache is (should be >90% unless you've just booted)

ext_cache_obj:ec0:accesses:32047/s <----- number of times the cache was accessed per sec

ext_cache_obj:ec0:disk_reads_replaced:3768/s <---- how many disk reads were not replaced by the cache.

ext_cache_obj:ec0:hit:17898/s <---- ave number of hits per sec (add to the number of misses below to get total)

ext_cache_obj:ec0:hit_normal_lev0:16517/s <---- this is 'normal' data hits. here you can see most of the hits are 'normal' data

ext_cache_obj:ec0:hit_metadata_file:907/s <--- this and the next few are the types of hit

ext_cache_obj:ec0:hit_directory:42/s

ext_cache_obj:ec0:hit_indirect:435/s

ext_cache_obj:ec0:total_metadata_hits:1385/s

ext_cache_obj:ec0:total_metadata_misses:434/s <--- if this is high relative to the total operations, you should consider metadata cache mode

ext_cache_obj:ec0:miss:14149/s <---- number of misses per sec

ext_cache_obj:ec0:miss_metadata_file:85/s <---- this and the next few are the types of miss

ext_cache_obj:ec0:miss_directory:1/s

ext_cache_obj:ec0:miss_indirect:347/s

ext_cache_obj:ec0:hit_percent:55% <--- % hits (hits / miss)

ext_cache_obj:ec0:inserts:426/s

ext_cache_obj:ec0:inserts_normal_lev0:337/s

ext_cache_obj:ec0:inserts_metadata_file:49/s

ext_cache_obj:ec0:inserts_directory:6/s

ext_cache_obj:ec0:inserts_indirect:39/s

ext_cache_obj:ec0:evicts:11/s <---- this indicates the number of blocks evicted (ie not accesses frequently) If this is very high you *may* need more flash caches